1. Field of the Invention
The present invention relates to optical isolators, and particularly to an optical isolator provided with a beam displacer that eliminates any undesired displacement of a light beam propagating through the isolator.
2. Description of Prior Art
Optical isolators are key elements in fiber optic communications systems in which optical signals are generated by lasers. However, if a laser used in a transmitter is subject to undesired optical reflection, this can result in optical wavelength jitter, laser output intensity noise, and uncontrolled optical power modulations. Accordingly, an optical isolator is used in a fiber optic communication system to block reverse transmission of optical signals to a laser transmitter while providing low attenuation of forward transmission.
Referring to FIG. 1, a conventional optical isolator 100 disclosed in U.S. Pat. No. 5,446,813 comprises first and second collimators 110, 120, and an isolating unit 130 disposed between the first and second collimators 110, 120. The isolating unit 130 comprises first and second birefringent wedges 131, 132, an optical rotator 133, and a magnetic tube 134 retaining the optical rotator 133 therein. The first and second birefringent wedges 131, 132 are fixed to opposite ends of the optical rotator 133 with adhesive. The first and second collimators 110, 120 are disposed at opposite sides of the isolating unit 130.
According to Snell""s law, when a light beam travels through a different medium, a displacement will generally occur in its direction of propagation. Referring to FIG. 2, when a light beam travels through the isolating unit 130, a slight displacement occurs. The displacement is designated as D, and occurs because of a difference between refractive indices of air and the isolating unit 130. In practice, the displacement D must be adjusted manually by using a microscope, so that light beams can be directed from the isolating unit 130 to the second collimator 120. If the displacement D is not eliminated, two polarized light beams 120A, 120B cannot be efficiently coupled concurrently into the second collimator 120. The result is increased insertion loss. Furthermore, the first and second birefringent wedges 131, 132 are fixed to opposite ends of the optical rotator 133 with adhesive. It is therefore impossible to further adjust the predetermined angle between transmission axes of the first and second birefringent wedges 131, 132.
In view of the above, an object of the present invention is to provide a method for manufacturing an optical isolator which can obviates the need for using a microscope to collimate light beams exiting from an isolating unit to a second optical collimator.
Another object of the present invention is to provide an optical isolator having two birefringent wedges respectively fixed to corresponding collimating lenses to ensure correct orientation of transmission axes of the birefringent wedges.
In order to achieve the objects set out above, an optical isolator in accordance with the present invention comprises a first optical collimator, an optical isolating unit, a second optical collimator and a tubular housing. The first optical collimator comprises an input optical fiber and a first collimating lens. The second optical collimator comprises an output optical fiber and a second collimating lens. The optical isolating unit comprises an optical rotator, first and second birefringent wedges, a pair of beam displacers, and a magnetic tube. Each beam displacer creates an offset of a light beam that effectively eliminates any displacement of the light beam that might otherwise occur. The beam displacers are fixed to respective opposite ends of the optical rotator. The first and second birefringent wedges are respectively fixed to inmost end surfaces of the first and second collimating lenses by conventional means. The magnetic tube encloses protruding portions of the first and second collimating lenses, the first and second birefringent wedges, the beam displacers and the optical rotator therein.
In assembly, relative orientations of the first and second optical collimators are adjusted such that optimized insertion loss and isolation are achieved.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: